RNA helicases of the DexH/D family play an essential role in viral replication and cellular RNA metabolism, including central functions in RNA splicing, translation and regulation of gene expression. Despite the importance of these proteins, their RNA helicase activity has not been subjected to enzymological study. Basic knowledge of cellular metabolism is therefore constrained by our limited understanding of reaction mechanism by motor proteins in the RNA helicase family. To address this problem, mechanistic studies have been initiated on two viral DexH/D proteins: NPH-II from Vaccinia and NS3-4A from Hepatitis C Virus (HCV). The NPH-II protein is show to be a processive, directional RNA helicase with specific roles for both the binding and hydrolysis of ATP. Having established qualitative features of NPH-II activity, this proposal aims to use direct and stopped flow kinetic measurements to determine the quantitative kinetic parameters such as translocation rates, reaction step size, processivity, helicase binding, ATP binding and hydrolytic rate constants that describe the framework for catalytic activity of this prototypical RNA helicase. In addition, the determinants for molecular recognition between RNA and helicase will be established. To determine if these findings are general and to extend the helicase studies to a viral system that poses a grave threat to public health, a complementary mechanistic framework will be developed for the HCV protein NS3-4A. This helicase will also be the subject of biophysical analyses to establish the link between cycles of ATP hydrolysis and translocative steps of the helicase protein. The mechanistic information will facilitate meaningful studies on HCV inhibitors and antiviral therapies and, given the availability of a crystal structure will set the stage for structure/function work on mutants of the NS3-4A protein. The NS3-4A protein is also useful because it is a promising candidate for novel mechanistic studies on helicase function in membrane-bound states and in the context of complex macromolecular machines.